System and method for additive manufacturing

ABSTRACT

A method for forming a component includes providing a first layer of a mixture of first and second powders. The method includes determining the frequency of an alternating magnetic field to induce eddy currents sufficient to bulk heat only one of the first and second powders. The alternating magnetic field is applied at the determined frequency to a portion of the first layer of the mixture using a flux concentrator. Exposure to the magnetic field changes the phase of at least a portion of the first powder to liquid. The liquid portion couples to at least some of the second powder and subsequently solidifies to provide a composite component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/190,460, filed on Feb. 26, 2014, which claims the benefit of U.S.Provisional Application No. 61/833,020 filed on Jun. 10, 2013, U.S.Provisional Application No. 61/868,625 filed on Aug. 22, 2013, U.S.Provisional Application No. 61/885,806 filed on Oct. 2, 2013, U.S.Provisional Application No. 61/896,896 filed on Oct. 29, 2013, U.S.Provisional Application No. 61/898,054 filed on Oct. 31, 2013, and U.S.Provisional Application No. 61/938,881 filed on Feb. 12, 2014. Theentire disclosures of each of the above applications are incorporatedherein by reference.

FIELD

The present disclosure relates to a system for additive manufacturingand, more particularly, to a system and method of selectively sinteringa mixture of powders using micro-induction sintering.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Current processes for producing high purity bi-component materials, suchas refractory metal parts, include powder and ingot metallurgy. Theingot metallurgy process begins with selecting and blending suitablepowders, pressing into bars, and sintering. An electron beam or plasmaor arc furnace is used to melt the bar in an inert atmosphere and coolit into an ingot. The melting can be done in multiple steps. Electronbeam melting and re-melting removes impurities to produce an essentiallypure ingot. The ingot is thermo-mechanically processed and further coldor hot worked as needed (or cold worked with intermediate annealing) toproduce a desired shape such as plate, sheet, rod or fabricated.Components may also be machined directly from ingots.

The sintering process consumes a significant amount of furnace time, butit is required to provide sufficient mechanical strength in the bars andis a preliminary deoxidation step for the refractory metal powder, suchas tantalum. The bars are usually electron beam-melted under a hardvacuum to remove impurities. The electron beam melting process can alsoconsume a significant amount of furnace time and power.

Laser additive manufacturing is a direct deposition process that uses ahigh power laser and powder feeding system to produce complexthree-dimensional components from metal powders. The high power laserand multi-axis positioning system work directly from a CAD file to buildup the component using a suitable metal powder. This process is similarto conventional rapid prototyping techniques such as stereolithography,selective laser sintering (SLS), and laser welding. Laser welding wasdeveloped to join two components or to fabricate an article integral toa component. Such a laser process has been used to manufacture near-netshape titanium components for the aerospace industry.

To date, an additive manufacturing process does not exist for highertemperature bi-component refractory and tooling materials, orbi-materials, where one material is sensitive to the high energy appliedby the laser. The application of a directed high energy beam to a powdermixture can cause damage to one or more of its constituent components.In this regard, this energy can cause undesired phase and structuralchanges within one or both of these component materials. As an example,superconductors encapsulated into a metal matrix are highly sensitive tothe application of a laser induced energy which may destroy theirsuperconducting capabilities. Additional problems can occur when theapplication of a laser to a powder mixture leads to undesired chemicalreactions between the materials. As such, there is a need for anadditive manufacturing system which overcomes some of the deficiencieslisted above and allows for a more creative combination of materials.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

A method for forming a component includes providing a first layer of amixture of first and second powders. The method includes determining thefrequency of an alternating magnetic field to induce eddy currentssufficient to bulk heat only one of the first and second powders. Thealternating magnetic field is applied at the determined frequency to aportion of the first layer of the mixture using a flux concentrator.Exposure to the magnetic field changes the phase of at least a portionof the first powder to liquid. The liquid portion couples to at leastsome of the second powder and subsequently solidifies to provide acomposite component.

According to the present teachings, a method for forming a component ispresented. The method includes selectively applying a magnetic field toa first portion of a layer of a powder mixture to selectively melt thefirst portion.

According to further teachings, a method of forming a component from amixture of first and second particles is presented. The method includesselectively applying a magnetic field to a first portion of the powdermixture. The magnetic field is applied at a frequency and field strengthto cause melting of the first particles within the first portion of thepowder.

According to another teaching in the present disclosure, a method forforming a component is presented. The method includes providing a layerof a mixture of first and second particles, the first particles having afirst particle size distribution. The method includes applying a highfrequency magnetic field to a first portion of the layer. The highfrequency magnetic field has a plurality of frequencies between a firstfrequency corresponding to a first portion of the first sizedistribution, and a second frequency corresponding to a second portionof the size distribution.

According to another teaching in the present disclosure, a method offorming a component from a mixture of first and second powder materialsis disclosed. The first powder material has a first resistivity and thesecond material has a different second resistivity. The method includesapplying a high frequency magnetic field to a first portion of thepowder mixture so as to cause at least a portion of the particles in thefirst powder material to melt.

According to another teaching of the present disclosure, a method offorming a component from a mixture of first and second powder materialsis disclosed. The method includes forming a first layer of a material ofthe mixture of particles. Next, a magnetic field is applied to a firstportion of the first layer so as to cause a first set of particles inthe first portion to melt. Next, a second layer of the mixture ofparticles is disposed over and in contact with the first layer. A secondmagnetic field is selectively applied to a second portion of the secondlayer to cause a second set of particles in the second layer to melt.When the sintering is completed, the first portion is coupled to thesecond portion.

According to another teaching, the method above includes applying aplurality of magnetic fields having between a first frequency and firstpower level, and a second frequency and second power level to the firstportion of the first layer so as to effect the melting of powderparticles having one of varying size and resistivity.

According to another teaching, a system for forming a component isprovided. The system includes a bed configured to hold a first layer ofa mixture of metal powder, and a magnetic flux concentrator configuredto apply a magnetic field at a frequency and field strength necessary tomelt a first portion of the powder within the first layer.

The system further includes a mechanism for applying a second layer of asecond mixture of material in contact with the first layer. The systemthen applies a second magnetic field to the second layer to melt asecond portion of the second layer, where the second portion is fused tothe first layer when the second magnetic field is removed. Further areasof applicability will become apparent from the description providedherein. The description and specific examples in this summary areintended for purposes of illustration only and are not intended to limitthe scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 represents a schematic representation of the additivemanufacturing system according to the present teachings;

FIGS. 2a-2d represent the application of micro induction heating toparticles according to the present teachings;

FIG. 3 represents a graph representing power transfer factors formaterials subjected to micro induction heating;

FIGS. 4a-5c represent micro induction sintering according to the presentteachings;

FIGS. 6a-6c represent graphs showing operating parameters for the microinduction heating system according to the present teachings;

FIGS. 7a-7c represent the formation of a fiber reinforced metal matrixmaterial according to the present teachings;

FIG. 8 represents fibers which can be used in the process shown in FIGS.7a -7 c;

FIG. 9 represents the simulated application of high frequency magneticfields to powders having materials having varying size distributions;

FIG. 10 represents a flow chart describing the formation of a laminarcomponent;

FIGS. 11a-11c represent the formation of a bi-material componentaccording to the present teachings;

FIG. 12 represents a flux concentrator according to the presentteachings; and

Tables 1-5 represent various material properties of powder materialsused according to the present teachings.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

FIG. 1 depicts a system 10 for producing a component 11 using anadditive micro-inductive sintering method in which layers of a mixtureof powder 12 are consolidated. A powder holding tray or bed 13 retainsthe mixture of powder 12. Disposed above the tray 13 is a gantry 15which supports a magnetic flux concentrator 17. A power supply and waveform generator 19 provides energy to magnetic flux concentrator 17 toapply a distinct, high frequency alternating magnetic field toselectively heat individual particles of the mixture of powder 12.

System 10 further includes a mechanism 21 having a pouring spout andleveling mechanism that recursively places layers of the mixture ofpowder 12 over previously consolidated portions of the mixture of powder12. Also shown is a sensor 25 that detects information such as thetransfer of energy to the mixture of powder and the degree ofconsolidation.

Unlike laser or electron beam based additive manufacturing techniques inwhich the metal powder is heated indiscriminately by an external energysource, the system 10 uses micro-induction sintering for the selectiveheating of individual particles by tailoring the frequency of an appliedmagnetic field. During micro-induction sintering, the system 10 appliesa localized high frequency magnetic field produced over an upper surfaceof the powder bed using the flux concentrator 17. System 10 causes arapid heating of individual particles followed by a rapid cooling of theconsolidated material due to a decoupling of the high frequency magneticfield from the melted particles that no longer exhibit the particle sizebeing excited.

Heating of metallic particles within the mixture of powders 12 byinduction is a result of both Joule heating due to eddy currents innon-magnetic metallic particles and hysteresis loss in magneticparticles, both of which result from the application of a high frequencymagnetic field. For non-magnetic metals, eddy currents flow within acertain distance from the surface of the material.

$\begin{matrix}{\delta = \sqrt{\frac{\rho}{\pi \; f\; \mu}}} & \lbrack 1\rbrack\end{matrix}$

The distance within the metal at which the eddy current is reduced toapproximately 37% of the value at the surface is called the skin depth δand can be written as where ρ is the resistivity and μ is thepermeability of the material, and f is the frequency of the magneticfield. In order to completely heat a metal particle by induction, theparticle is immersed in a high frequency magnetic field such that theskin depth is approximately one half the diameter of the particle.Generally, high power transfer to the particle occurs near a diameterapproximately four times the skin depth for simple geometries such asplates and cylinders with the magnetic field parallel to the axis of thepart. For spheres, it is expected this ratio of the particle diameter tothe skin depth would be higher.

FIG. 2a depicts the heating of a single particle by induction. Thediameter of the particle is approximately 2 δ. In this case, the eddycurrents penetrate deep into the particle and bulk heating of the entireparticle occurs by induction and heat transfer through the particle at asingle frequency. Due to particle size distributions as well as particleshape anomalies, a band of frequencies is preferred to sinter a mixtureof powders 12.

FIG. 2b depicts when the diameter of the particle is much larger than 8.Due to the directional nature of the magnetic field, only a portion ofthe particle outer skin is melted corresponding to the skin depth δ. Forgiven resistivity and particle sizes, the melting can occur either onlyat the surface or through an entire circular layer of the particle (seeFIG. 2c ). A band of frequencies can be applied to correspond to varioushemispherical diameters D₁-D₅ at different circles of the sphere. In theexamples depicted in FIGS. 2b and 2c , the frequencies applied can varyfrom 1 to about 5 times the frequency calculated to melt the largestdiameter for a given particle having a specific resistivity.

FIG. 2c depicts when a band of frequencies are applied to melt a set ofcylindrical disks through the particle. In this example, D₁-D₅correspond to frequencies which form a skin depth of approximately 2 δ.Optionally, to melt the particle, the frequency band of the magneticfield F need not completely cover each of the frequencies correspondingto diameters D₁-D₅. Melting of the whole or a sufficient portion of theparticle can occur by applying frequencies corresponding to skin depths,for diameters D₂ and/or D₃, where melting the entire particle, orsurface of the particle occurs through heat conduction. The heat energyrequired to melt the remainder of the particle transfers through theparticle via normal heat diffusion processes.

In FIG. 2d , the skin depth is much larger than the diameter of theparticle and the eddy currents largely cancel in the particle. In thiscase, the particle does not couple well to the alternating magneticfield and the material absorbs very little power. It is envisioned thesystem would use frequencies such that the heating would be completed asshown in FIGS. 2a-2c . There is little heating of the particle atfrequencies depicted in the case shown in FIG. 2 d.

For simple shaped (e.g. flat or cylindrical) materials placed in auniform alternating magnetic field, the power absorbed by the particleP_(w) can be:

$\begin{matrix}{P_{w} = {{\frac{\rho}{\delta}{AKH}^{2}} = {{AKH}^{2}\sqrt{\pi \; f\; \mu \; \rho}}}} & \lbrack 2\rbrack\end{matrix}$

Where p is the resistivity of the material, δ is the skin depth, A isthe particle surface area exposed to the magnetic field, K is a powertransfer factor that depends on particle geometry, and H is the magneticfield strength. It should be noted that resistivity changes as afunction of temperature and, as such, it is envisioned that the P_(w)may be adjusted through time depending upon changes in static anddynamic thermal conditions during the formation of a component. It ispossible to calculate the power absorbed by a given metallic particle inan induction heating process using modern finite element analysismethods. As a rule of thumb, with a fixed resistivity, magneticpermeability and particle dimensions, the power absorbed by the particlein an induction heating process increases with increasing frequency andmagnetic field strength.

The only ill-defined quantities are A and K, which describe how well thehigh frequency magnetic field couples to the individual particle. Forany given slice through an approximately spherical particle, d/δ can becalculated from the particle diameter at that slice. The power transferfactor K, on the other hand, depends on the “electrical dimension” ofthe portion of the particle being heated, which is defined as the ratioof the diameter of the particle to the skin depth, d/o.

FIG. 3 represents a graph representing power transfer factors formaterials subjected to micro induction heating. Power transfer factorsfor two cases of a plate and a cylinder are shown. Using the plategeometry as a crude model for roughly spherical particles, it is seenthat K approaches unity if the skin depth is much smaller than thethickness of the particle. For example, when d˜2δ, K is approximately0.8. The system utilizes the functional dependence of K(d/δ) fordetermining the appropriate frequency or frequencies for the selectiveheating of individual particles in composite material. The system 10utilizes two conceptual composite architectures with an emphasis on theselective heating of individual components of the composite componentduring the consolidation process. Accordingly, the selectivity of thesystem's micro-inductive sintering is based both on the size andmaterial properties of the particles in the powder.

FIGS. 4a-4c illustrate the application of micro-inductive sintering to amixture of two mono-sized dispersed metal powders. In FIG. 4a , thepowder mixture 12 consists of a first material 14 and a second material16 with approximately the same particle size or particle sizedistributions, but with different material properties. The resistivity pof the particles of the first material 14 is ten times greater than theresistivity of particles of the second material 16. Assuming that bulkheating of the particles occurs when 2 δ is equal to the diameter of theparticle, the induction frequency can be:

$f = \frac{4\; \rho}{\pi \; \mu \; d^{2}}$

where d is the diameter of the particle.

Thus, for a given particle size and magnetic permeability, the inductionfrequency to achieve bulk heating of a particle scales linearly with theresistivity of the material. In this case, the particles of the firstmaterial 14 can be selectively heated in bulk using an oscillatingmagnetic field with a frequency ten times smaller than that which wouldbe used to bulk heat the particles of the second material 16. This isillustrated in FIG. 4b , which explicitly shows the selective bulkheating of the particles of the first material 14 as indicated by doublecross-hatching. FIG. 4b depicts the heating of the particles of thefirst material 14 where the frequency of the magnetic field is set suchthat the skin depth is approximately one half the diameter of theparticle. The skin depth of the particles of the second material 16 isapproximately (10)^(0.5)˜3.2 times that of the first particle at thisfrequency. Since the skin depth in the second particle is much largerthan the particle diameter, there is very poor coupling to the highfrequency magnetic field and these particles are not heated directly byinduction. Note that the particles of the second material 16 are alsoheated in this process, but only by conduction and convection heatingwhich results from the induction heating of the particles of the firstmaterial 14. As such, only an outer portion 18 of the particles of thesecond material 16 are heated as depicted by double cross-hatching. Theselective sintering of powders that possess similar particle sizedistributions, but different materials properties can be used to informthe power levels and frequencies needed for micro-inductive sintering.

FIG. 4c represents a portion of a consolidated component 11 where thepreviously heated and melted particles of the first material 14 have nowcooled after completion of the selective sintering process. It should benoted that the isolated particles of the second material 16 remain asinclusions within the recently formed solid of the first material 14.Upon consolidation of the particles of the first material 14, theeffective domain size of the first material 14 increases such that thehigh frequency magnetic field tuned to the initial diameter of theparticles of the first material 14 no longer couples well to the firstmaterial 14. In this case, the effective particle size is much largerthan the skin depth at this frequency and the entire consolidated domainis heated only at the surface as previously described in relation toFIG. 4 b.

In one exemplary manufacturing method, the bed 13 of the mixture ofpowder 12 may be heated to a temperature near the melting temperature ofthe particles of the first material 14. Only the very low overalladditional energy needed to melt the powder 12 need be inputted into thepowder bed 13 by the flux concentrator 17 to selectively melt theparticles of first material 14. The additional energy is localized tothe active micro-inductive sintering zone near a gap 23 in the fluxconcentrator 17. For example, high frequency induction of eddy currentsin a metallic binder (particles of the first material 14) allows for theselective heating and subsequent consolidation of a ceramic/metal matrixcomposite without the associated heating and degradation of the ceramicconstituent (particles of the second material 16). This makes itpossible to consolidate composites composed of very heat-sensitiveceramic particles (e.g., superconducting materials).

The coupling and de-coupling of the high frequency magnetic field basedon the domain size of the metallic material is a unique and novelfeature specific to the micro-inductive sintering process of the presentdisclosure. This property allows for real-time diagnostics of themicro-inductive sintering consolidation process through the monitoringof the forward and reflected power to the powder bed. In addition, thisprocess allows for the rapid and automatic de-coupling of the externalheat source (i.e. the high frequency magnetic field) upon consolidationof the particles. This is a desirable control feature in theconsolidation of heat sensitive materials or composite materials thatmay degrade upon exposure to elevated temperatures.

As previously stated, the selectivity of the system's micro-inductivesintering is based both on the size and material properties of theparticles in the powder. The metal powder shown in FIG. 5a consists of abimodal distribution of first particles 22 and second particles 24. Thesecond particles 24 are the larger of the two particles havingapproximately twice the diameter of the smaller first particles 22.Again, either the smaller or larger particles may be selectively heatedby the induction frequency, where it is seen that the inductionfrequency varies as a function of size. Thus, a twofold increase inparticle size implies a fourfold decrease in the frequency of theoscillating magnetic field necessary to achieve bulk heating, assumingthe optimum “electrical dimension” for heating the particles was equalto 2.

FIG. 5b illustrates the bulk heating of the smaller first particles 22and the surface heating of an outer portion 26 of larger secondparticles 24 which is characteristic of the micro-inductive sinteringprocess. Using a narrow bandwidth of fixed frequencies, completeconsolidation of the effected region is shown in FIG. 5c . As in theprevious example, upon consolidation of the particles, the effectivedomain size of the material increases and the high frequency magneticfield tuned to the initial diameter of the smaller first particles 22becomes de-coupled from the consolidated material and the entire domainis heated by induction only at the surface.

In the composite architectures previously described, the frequency ofthe induction heating process is used to selectively heat specificcomponents of the composite based on the physical or materialscharacteristics of the powder. In the prior example, the small firstparticles 22 are selectively heated by induction, which results in theconsolidation of the material. By changing the frequency or spectrum ofthe magnetic field, however, the large particles could have beenselectively heated by induction, which may lead to an improved densityof the final part. In practice, the specific sintering characteristicsof the material and the desired material properties of resultantmaterial will determine the micro-inductive sintering frequencyspectrum. Overall, the micro-inductive sintering approach allows forenhanced control of the densification process by targeting smallparticles, or large particles that can be partially or entirely melted.This control adds another tool in the toolbox for the effectiveconsolidation of powders suitable for use in additive manufacturing.

By selective application of the magnetic fields, micro inductionsintering produces complex parts and components directly from advancedmetal and ceramic/metal matrix composite powders. The micro-inductivesintering process, however, is not without limitations imposed by thesystem electronics, the magnetic properties of the magneto-dielectricmaterial used to fabricate the flux concentrator 17, the specificsintering characteristics of the metallic powders, and the fundamentalphysics of induction heating. In general, the micro-inductive sinteringprocessing is preferable within the following operational parameters:

1) Materials with electrical resistivity between 1 μOhm cm and 200 μOhmcm.

2) Powders with particle sizes between 1 μm and 400 μm.

3) Flux concentrator induction frequencies between 1 MHz and 2000 MHz.

The operative phase space for the bulk and surface heating of powders byhigh frequency induction can be determined. FIGS. 6a and 6b show theoperative phase space for the micro-inductive sintering process with afixed particle diameter of 100 μm (FIG. 6a ) and with a fixedresistivity (FIG. 6b ) assuming an “electrical dimension” of 2. Thehighlighted area between the surface and bulk heating curves indicatesthe parameter space in which the micro-inductive sintering processoperates. Any material that falls out of this range is not a preferredcandidate for the micro-inductive sintering process based on theoperational parameters listed above. An alternative representation isshown in FIG. 6c , which illustrates the operational frequency of themicro induction sintering process as a function of particle size andresistivity. There are three primary operational frequency bands shownin the Figure:

High Frequency (HF)—frequencies less than 30 MHz and greater than 0.1MHz.

Very High Frequency (VHF)—frequencies greater than 30 MHz and less than300 MHz.

Ultra High Frequency (UHF)—frequencies greater than 300 MHz and lessthan 3 GHz.

The vast majority of materials used in additive manufacturing processespossess particle size distributions ranging between 50 μm and 150 μmwith electrical resistivities less than 100μΩ cm. This operational spaceis highlighted by the box outline in FIG. 6c , which shows that mostmaterials can be heated by the MIS process in the VHF and UHF bands. Amaterial that falls within the lowermost, non-hatched region shown inFIG. 6c may not be a practical candidate for the MIS process based onthe operational parameters listed above.

One example of a material formed using the micro-inductive sinteringsystem is Tungsten Carbide/Cobalt (WC—Co) composites which are usedextensively for machine tools, metal cutting, dies, and wear resistantcoatings. These materials are fabricated with a fine dispersion of WCparticles in a Co matrix (5% to approximately 30% by weight) anddemonstrate high strength, high toughness and high hardness. WC—Co partsare conventionally formed using powder metallurgy and sintering atapproximately 1700K. Cobalt serves as the binding metal in thesecomposites and a uniform dispersion is critical to the overallperformance of the composite material.

According to the present teachings, a WC and Co composite of powderedmaterial is formed with average component domain sizes of 1 μm and 5 μm,respectively. This powder has morphology similar to that shown in FIG.5a , with the smaller first particles 22 being WC and the larger secondparticles 24 being Co metal. The materials properties of WC and Corelevant to the micro-inductive sintering process are shown in Table 1.In order to consolidate this material by high frequency induction, thecobalt matrix is selectively melted, which will then cause densificationof the composite through liquid phase sintering. As seen in Table 1,cobalt is magnetic with a relative permeability on the order of 100 anda Curie temperature of approximately 1400K. Thus, heating of the Coparticles will occur due to Joule heating and hysteresis loss attemperatures less than the Curie temperature. Above the Curietemperature, the only heating mechanism is via eddy currents in themetallic domains. The Micro-Induction Sintering of WC—Co composites area complicated but feasible process involving a wide bandwidth,high-frequency, flux density that is primarily coupled to the cobaltmetal.

During the micro-inductive sintering consolidation process, theeffective domain size of the cobalt increases which, as discussedpreviously, will de-couple the high-frequency magnetic field from thematerial. The WC particles, on the other hand, do not change in sizeduring the process. Additionally, the size and electricalcharacteristics of WC are not effectively heated directly by thehigh-frequency magnetic field. Table 2 shows calculations for the bulkheating frequency of WC and Co particles as a function of temperatureand domain size.

The high resistivity of WC prevents this material from being heated byinduction. A 5 μm WC particle, for example, would require an inductionfrequency of approximately 8 GHz for bulk heating. Although envisionedherein, these frequencies are impractical given the present state ofexisting flux concentrator materials and power supplies. Formicro-inductive sintering process frequencies less than 100 MHz, the WCparticles are magnetically “invisible” to the high frequency magneticfield and are only heated indirectly through contact with the cobalt.

Based on predictive performance within the envelope of the operationalparameter phase space shown in FIGS. 6a, 6b, and 6c , TungstenCarbide/Cobalt is a good candidate for micro-inductive sintering-basedparticle formation. The strength, toughness, and hardness of WC—Cocomposites make the material highly desirable for many manufacturingapplications.

It may also be desirable to construct components as fiber reinforcedmetal composites. Fiber reinforced metal composites are made from avariety of materials including carbon, silicon carbide and aluminafibers in a metal matrix, typically aluminum, titanium or magnesium. Theaddition of the fibers can dramatically improve the performance of themetal by increasing strength, stiffness, dimensional stability and heatresistance. The defense, automotive, and aerospace markets have a stronginterest in these materials in order to deliver higher performance,higher reliability and/or lighter weight components. Key applicationsareas for these materials today include engine cylinder blocks andpistons, brakes, and dimensionally stable structural components for usein space.

The micro-inductive sintering process can provide an improved method forconsolidating carbon fiber-reinforced composites, particularly foraluminum and magnesium-based matrix materials. Tables 3 and 4 show thebulk heating frequency of Al, Mg, and representative carbon fiber as afunction of particle size. Since the micro-inductive sintering processis based on powder metallurgy, the challenge of getting a homogeneousdistribution of the carbon fibers during the liquid phase infiltrationstep is eliminated. As shown in FIGS. 7a-7c , to produce these materialsthe metal powder and carbon fibers are combined using conventionalpowder milling methods to give a uniform distribution of the carbonfiber reinforcement with the metal matrix.

A further advantage of the micro-inductive sintering process is that itallows for the selective heating and rapid cooling of the metal matrix.As discussed previously, the induction frequency spectrum can be setsuch that the skin depth is approximately one-half of the particle sizedistribution of the metal matrix. Upon consolidation of the metalparticles, the effective domain size increases and the micro-inductivesintering frequency spectrum will no longer couple well to the largermetallic domain. In a sense, this heating process is self-limiting inthat it becomes inefficient as soon as the particles in the metal matrixcoalesce. Aluminum and Al—Mg alloys possess relatively low meltingpoints, which in combination with the very high vapor pressure of Mg,can lead to the loss of material during the additive manufacturingprocess if the local temperature is too high. The materials propertiesof Al, Mg, and representative carbon fiber relevant to themicro-inductive sintering process are shown in Table 3.

The high vapor pressure of Mg near and above its melting point can leadto a significant change in the composition of high strength Al—Mg fiberreinforced alloy parts fabricated using alternative additivemanufacturing technologies.

The Micro-Induction Sintering of carbon fiber-composites ischaracterized by a wide bandwidth, high-frequency, flux density that iscoupled to the metal matrix. During the micro-inductive sinteringconsolidation process, the effective domain size of the metal matrixincreases which, as discussed previously, will de-couple the highfrequency magnetic field from the material. The high resistivity ofcarbon, along with the small dimensions of the fibers, render the carbonfibers invisible to the induction heating process at frequencies lessthan 100 MHz. The induction frequency for aluminum and magnesiumparticles with the appropriate particle size distribution is between 2MHz and 20 MHz. Similar to the WC particles in WC—Co composites, thecarbon fibers are heated only through the contact with the metal matrix.

The composite starts out as a homogeneous mixture of powder metal matrix(the first material 14) and reinforcing fibers (the second material 16).The matrix material is heated to sintering temperatures by ahigh-frequency magnetic field tuned to match the particle size of themetal powder as shown in FIG. 7b as indicated by double cross-hatching.Upon consolidation, the metal matrix decouples from the oscillatingmagnetic field and rapidly cools. The resulting composite is aconsolidated material with uniform distribution of reinforcing fibers asshown in FIG. 7 c.

As shown in FIG. 8, the carbon reinforcement fibers can take on avariety of forms from single walled carbon nanotubes with a diameter of1 nm, to multi-walled carbon nanotubes with diameters of 10-100 nm tocarbon fibers with diameters of 1-10 μm. Table 5 shows a potential listof candidate materials/applications. While certain of the candidatematerials considered were eliminated due to technical suitability forthe micro-inductive sintering system, some were eliminated simplybecause other manufacturing methods exist for the materials and thusshould be considered for the applications of the techniques disclosedherein. It is envisioned the techniques described herein may be usedwith all of these materials.

It is expected that the micro-inductive sintering system will showadvantages over laser system with reflective metals (e.g. aluminum andniobium) as the optical reflection of the laser from the metal drives aneed for much higher powered lasers for sufficient heat absorption toconsolidate the metal. Optical reflectivity has no effect on themicro-inductive sintering system's interaction with materials, andtherefore the micro-inductive sintering approach may prove to be moreefficient with these materials.

In addition to continued exploration of the chosen materials, it isexpected that new material compositions will be developed which areeither targeted for use with the micro-inductive sintering method(specifically to take advantage of the unique capabilities of theprocess) or that naturally have benefit from the techniques and methodswhich will be described in detail below.

FIG. 9 represents the simulated application of high frequency magneticfields to the mixture of powder 12. As shown, the mixture of powder 12includes the particles of the first material 14 having a first particlesize distribution 30 and the particles of the second material having asecond particle size distribution 32. Corresponding to the first andsecond particle size distributions 30, 32 are a pair of magnetic fieldfrequency distributions 34, 36. The magnetic field frequencydistributions 34, 36 correspond to a group of frequencies designed tomelt the particles of the first and second materials.

As described above, the system 10 can apply a frequency spectrum 37 ofmagnetic fields ranging between vertical lines A and B or a frequencyspectrum 38 ranging between lines B and C to melt only one of the twomaterials. Alternatively, the system 10 can apply a spectrum 39 rangingbetween lines A and C configured to melt a majority of both the particledistributions. The system can sequentially apply the magnetic fieldthrough the flux concentrator 17 as a frequency sweep. Alternatively,the signals to the flux concentrator 17 can be multiplexed and caninclude all or a portion of the frequencies within a spectrum.

FIG. 10 includes a flow chart describing the formation of a bi-materialcomponent 11 according to the present teachings and depicted in FIGS.11a-11c . The method for forming a component 11 includes producing alayer of powder mixture 12. Next, a magnetic field is applied to a firstportion 40 of a first layer 42 of the powder mixture 12 to selectivelymelt the first portion 40 of the powder mixture 12. Preferably, thepowder mixture 12 is formed of particles of the first and secondmaterials 14, 16 having mean diameters between 1 μm and 400 μm, andresistivity between 1 μΩ cm and 200 μΩ cm. Based on the sizedistribution and resistivity of the particles of the first material 14,the absorbed power changes the phase of the effected particles of thefirst material to liquid. The first liquid material is solidifiedautomatically (FIG. 11b ) as the melted particles no longer exhibit theproper dimensions for bulk heating by the applied magnetic field.Alternately, solidification may be achieved by removing the magneticfield.

As shown in FIG. 11c , after the portion 40 of the first layer 42 of thepowder mixture 12 has been solidified, a second layer 46 of mixture isdisposed over the first layer 42. Second portions 48 a and 48 b of themixture are then melted and solidified using the concentrated magneticfield. Layers of mixtures are recursively added and sintered to form thecomponent. Optionally, the system 10 can apply a magnetic field, havinga frequency between about 1 MHz and about 2.0 GHz, to the first layer ofthe powder mixture to melt at least a skin layer of a portion of thefirst material.

FIG. 12 is a schematic representation of flux concentrator 17 shown inFIG. 1. The flux concentrator 17 can have a horse-shoe shape having acurved body 50 which supports an inductive coil (not shown). Taperedarms 52 extend from curved body 50 to concentrate the magnetic field ata centralized location namely, gap 23.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A method for forming a component comprising:providing a first layer of a mixture of first and second powders;determining a frequency of an alternating magnetic field to induce eddycurrents sufficient to (i) bulk heat only the first powder and bulkheating of particles of the first powder occurs when a diameter of eachof a plurality of particles of the first powder is at least equal to 2δfor the respective particle of the first powder, wherein:$f = \frac{4\; \rho}{\pi \; \mu \; d^{2}}$ where d is the diameterof a respective particle of the first powder, ρ is the resistivity ofthe material of a respective particle of the first powder, f is thefrequency of the alternating magnetic field, and μ is the magneticpermeability of the material of the respective particle of the firstpowder and (ii) surface heat particles of the second powder; andapplying the alternating magnetic field at the determined frequency tothe first layer of the mixture using a flux concentrator, whereinexposure to the magnetic field changes the phase of at least a portionof the first powder to liquid by bulk heating the particles of the firstpowder and heating the particles of the second powder such that outerregions of the particles of the second powder change phase to liquidwhile inner regions of the particles of the second powder remain solid.2. The method of claim 1, wherein two or more particles of the firstpowder combine to form a consolidated material after the two or moreparticles change to the liquid phase, the consolidated material having asize de-coupling the magnetic field from the consolidated material. 3.The method of claim 1, wherein surface heating of each particle of thesecond powder occurs when 2δ for the respective particle of the secondpowder is less than a diameter of the respective particle of the secondpowder.
 4. The method of claim 1, wherein determining the frequency ofthe alternating magnetic field is based on a dimension of the particleswithin the first powder.
 5. The method according to claim 4, wherein thefirst powder has a first mean particle diameter and the second powderhas a second mean particle diameter which is different from the firstmean particle diameter.
 6. The method of claim 1, wherein determiningthe frequency of the alternating magnetic field is based on aresistivity of particles with the first powder.
 7. The method accordingto claim 6, wherein the first material has a first resistivity and thesecond material has a second resistivity different than the firstresistivity.
 8. The method according to claim 1, wherein the firstpowder includes particles from a first material and the second powderincludes particles from a different second material.
 9. The methodaccording to claim 8, wherein the first material is a metal and thesecond material is carbon fiber.
 10. The method according to claim 8,wherein the first material is WC and the second material is cobalt. 11.The method according to claim 8, wherein the first material is a metaland the second material is a ceramic.
 12. The method according to claim1, further comprising applying a second layer of powder material overthe first layer of powder material and applying the magnetic field tothe second layer.
 13. The method according to claim 1, furthercomprising ceasing to apply the magnetic field to solidify the liquidportion and couple a portion of the first powder with portions of thesecond powder.